This invention relates to improvements in a PCM recording and reproducing system and more particularly to means for producing phase signals serving to control the speed of travel of the PCM record tape involved.
Digital recording and reproducing systems utilizing a record disc, a magnetic record tape or the like have been previously employed for various purposes. In those systems the reproduction of the signal has been attended with an undesirable phenomenon called "jitter" or "wow and flutter." In order to eliminate this phenomenon, it has been a common practice to control the speed of travel of the particular PCM record tape by comparing phase signals appearing in the reproduced signal with a reference phase signal provided from an oscillator having high frequency stability. This measure yields remarkable results in PCM tape recorders intended to reproduce music with high fidelity because the wow and flutter are removed.
In conventional PCM tape recorders employing, for example, the stationary recording head, an audio signal in the analog form is first digitalized into a corresponding digital signal and then data including a predetermined number of samples of the digital signal are added with a frame synchronizing signal to form a frame. In this way the frames are formed one after another and are successively and repeatedly distributed to a plurality of parallel record tracks disposed on a magnetic recording tape to be speed-converted to a low speed PCM signal.
In a conventional speed control using phase comparison, phase signals are formed by sensing frame synchronizing signals resulting from a selected one of the record tracks and compared with the corresponding reference phase signals. In this phase comparison it is indispensable that the recorded signal be continuously reproduced while reproduced frame synchronizing signals be free from any distortion.
With the PCM recorders utilized, it is necessary to effectively accomplish the so-called splice editing of the PCM magnetic record tape. In splice editing, however, two cut pieces of the tape cannot be spliced to each other so as to cause the frame synchronizing signals on both tape pieces to coincide in phase with each other. This results in the discontinuity of the phase of the frame synchronizing signals. Therefore, reproduced phase signals have skips in phase. This means that, with splice editing effected, it is impossible to accurately control the speed of the tape with conventional phase comparison. Further, if the speed control effected by driving systems for moving the PCM magnetic record tape has been unstably distorted then a few seconds elapse until the speed control is returned back to its stable state. During the unstable speed control, the digital signal is not correctly reproduced. This has particularly caused a fatal disadvantage in the playback of musical signals.
Accordingly, it is an object of the present invention to provide a new and improved PCM recording and reproducing system preventing the speed control of a PCM magnetic record tape involved from being distorted even when the frame synchronization of the reproduced signal skips in phase.
It is another object of the present invention to prevent the speed control of a PCM magnetic record tape from being distorted after the splice editing of the tape.
It is still another object of the present invention to provide a new and improved PCM recording and reproducing system for reproducing a normal musical signal by preventing the musical signal from being distorted during the reproduction thereof.